Photonic devices are now being integrated on a same substrate as electronic circuits, such as CMOS circuits. The substrate material for such integration is typically silicon, either as bulk silicon or as a silicon on insulator structure. Many photonic devices can be formed in a patterned silicon layer provided over the substrate. For example, a waveguide core can be formed of silicon over a substrate, the latter of which can also be a silicon substrate, provided the silicon waveguide core is surrounded by a cladding having a lower index of refraction than the silicon of the core. Silicon dioxide is often used as a suitable cladding material as it has an index of refraction of about 1.45 compared to a refractive index of about 3.45 for silicon. Other materials can also be used for the waveguide core and cladding material provided there is a sufficient difference between the higher refractive index of the core and lower refractive index of the cladding.
However, some photonic devices must be formed of materials other than silicon, but which interface with silicon. A photo detector, for example, a PIN photodiode having P and N regions with an intrinsic region between, can be formed of silicon for the P and N regions and germanium or a silicon-germanium material (for a silicon-germanium material for use in a photo detector the germanium mole fraction should be at least 30%) for the intrinsic region. For purposes of discussion herein, whenever a silicon germanium material is discussed it will be presumed to have a germanium mole fraction of at least 30%. Germanium or silicon-germanium are often preferred for use in a photo detector for receiving light because they are more sensitive to light compared with silicon at the wavelengths greater than 1.1 μm.
When a germanium or silicon-germanium material is formed in contact with silicon, for example at the interface with the silicon P or N regions of a PIN diode, crystal lattice mismatch defects occur at the interface. For example, at a germanium-to-silicon interface, there is a 4.2% lattice mismatch. A lattice mismatch creates misfit dislocations which creates charge trap states at the interface. The trap states can interfere with the photoelectric conversion process by producing energy levels in the band gap of the photo detector which result in the production of dark current. Dark current reduces the overall signal to noise ratio of the photo detector and reduces its responsiveness to the photoelectric conversion of received light.
A photo detector structure and method of making it which lessens the effect of the misfit dislocations and corresponding trap states on the photo detector responsiveness to light would be desirable.